The invention is directed to improvements in air flow rate meters for internal combustion engines.
In a known air flow rate meter of this type (European Patent Document 0 054 887 C2), the inlet portion of the bypass conduit located upstream in the flow direction is embodied by a small tube that protrudes beyond the venturi portion of the main flow conduit and at its free end forms the entry opening for the bypass conduit. The length of the small tube is at a predetermined ratio to the inside diameter of the bypass conduit. By means of this small tube, an attempt is made to reduce the disadvantageous influence of turbulence at the upstream end of the bypass conduit on the measured value signal of the temperature-dependent resistor. To reinforce this effect, a flow straightening blade is used.
Upon deflection of the air flow direction upstream of the bypass conduit, for instance by 90.degree., as is often the case in air intake systems of internal combustion engines for reasons having to do with installation conditions, increased turbulence and a shift of the pressure profile causes markedly increased signal noise, which can no longer be adequately eliminated even with the flow straightening blade in the entry area of the bypass conduit.